The bush-border membrane is generally regarded as a mosaic of functional proteins (enzymes and transport carriers), perhaps assembled over a substratum of "structural" proteins. The general objective of this project is to take apart the pieces of this mosaic and, eventually, to reconstitute it. Specifically an effort is being made at characterizing at the molecular level a hypothetical unit formed by the specific binding sites involved in the active transport of sugars and amino acids, together with associated binding sites for sodium ion and the phenol, phloretin; and, perhaps, the disaccharidase, sucrase. Using small intestine from guinea pigs and other mammals, the brush-border membranes will be isolated and fractioned. The particles obtained will be separated according to size and/or charge, and characterized with regard to (1) presence of key enzymatic activities known to be membrane-bound (e.g., disaccharidases, peptidases, alkaline phosphatase) and (2) binding sites for certain substrates such as sugars and amino acids, as an index of the possible presence in the particles of the hereto hypothetical transport carriers (the correspondence between these proteins and the transport carriers will be monitored with parallel transport studies at the phenomenological level). Particles will be dissociated into their various protein components and analyzed using techniques such as polyacrylamide gel electrophoresis: this "fingerprinting" might help in elucidating the spatial distribution of proteins in the membrane, and their respective topological relations.